Soft and Softer Handover in Communication Netwoks

International Journal on Recent and Innovation Trends in Computing and Communication ISSN 2321 – 8169 Volume: 1 Issue: 6 558 – 562

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558 IJRITCC | JUNE 2013, Available @ http://www.ijritcc.org

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Soft and Softer Handover in Communication Netwoks

Dr. Jitendra Kumawat, Shrinth Tailor Amity School of Engineering Pratap University, Jaipur

& Technology, Amity University, Jaipur

Soft and softer handover are the CDMA specific handover

types implemented in the UMTS system and form one of

the most characteristic features of the revolutionary

WCDMA access method. In this paragraph the impact of

implementing this handover types on the system design is

discussed in detail and also the algorithms behind these

methods as described in 3GPP specification TR 25.922

are analyzed.

Figure: soft vs. softer handover

A soft or softer handover occurs when the mobile station

is in the overlapping coverage area of two adjacent cells.

The user has two simultaneous connections to the

UTRAN part of the network using different air interface

channels concurrently. In the case of soft handover the

mobile station is in the overlapping cell coverage area of

33 two sectors belonging to different base stations; softer

handover is the situation where one base station receives

two user signals from two adjacent sectors it serves.

Although there is a high degree of similarity between the

two handover types there are some significant differences.

In the case of softer handover the base station receives 2

separated signals through multi-path propagation. Due to

reflections on buildings or natural barriers the signal sent

from the mobile stations reaches the base station from two

different sectors. The signals received during softer

handover are treated similarly as multi-path signals. In the

uplink direction the signals received at the base station are

routed to the same rake receiver and then combined

following the maximum ratio combining technique. In the

downlink direction the situation is slightly different as the

base station uses different scrambling codes to separate

the different sectors it serves. So it is necessary for the

different fingers of the rake receiver in the mobile

terminal to apply the appropriate de-spreading code on the

signals received from the different sectors before

combining them together. Soft handover occurs in 5-10%

of the connections.

Due to the nature of the softer handover there is only one

power control loop active.


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For soft handover the situation is very similar in the

downlink direction. In the mobile station the signals

received from the two different base stations are

combined using

MRC Rake processing. In the uplink direction on the

other hand there are significant differences. The received

signals can no longer be combined in the base station but

are routed to the RNC. The combining follows a different

principle; in the RNC the two signals are compared on a

frame-by-frame basis and the best candidate is selected

after each interleaving period; i.e. every 10, 20, 40 or

80ms. As the outer loop power control algorithm

measures the SNR of received uplink signals at a rate

between 10 and 100Hz, this information is used to select

the frame with the best quality during the soft handover.

Note that although the situation is described for two

overlapping cells higher values are possible. As will be

discussed later, the number of cells the mobile station can

have simultaneous connections with is called the active

set size.

3G TR 25.922

This paragraph discusses the handover process as

described in TR 25.922 of the 3GPP specifications.

Basically the soft handover is composed of two main

functions:

- Acquiring and processing measurements

- Executing the handover algorithm

Before starting the in-depth analysis of these functions

some terms used for describing the handover process have

to be defined:

- Set: list of cells or Node B’s

- Active set: list of cells having a connection with the

mobile station

- Monitored set: list of (neighboring) cells whose pilot

channel Ec/I0 is continuously measured but not strong

enough to be added to the active set.

__Measurements

Accurate measurements of the Ec/I0 of the pilot channel

(CPICH) form the main input for obtaining the RRC

measurement report, necessary for making handover

decisions.

Mainly three parameters can be measured. Besides the

Ec/I0 of the CPICH also the received signal code power

(RSCP) and the received signal strength indicator (RSSI)

are measured. RSCP is the power carried by the decoded

pilot channel and RSSI is the total wideband received

power within the channel bandwidth. Ec/I0 is defined as:

RSSI

RSCP

I

EC =

0

(2.1)

It is important to apply filtering on the handover

measurements to average out the effect of fast fading.

Measurement errors can lead to unnecessary handovers.

Appropriate filtering can increase the performance

significantly. As long filtering periods can cause delays in

the handovers13, the length of the filtering period has to

be chosen as a trade-off between measurement accuracy

and handover delay. Also the speed of the user matters,

the slower the user equipment is moving the harder it is to

average out the effects of fast fading. Often a filtering

time of 200ms is chosen.

Other essential information needed during the so-called

intra-mode handovers – soft and softer handover – is

timing information. As the WCDMA network is of

asynchronous nature there exist relative timing

differences between the cells. To allow easy combining in

the Rake receiver and avoid delays in the power control

loops, the transmissions have to be adjusted in time. After

the UE has measured the timing difference between the


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CPICH channels of the serving cell and the target cell, the

RNC sends DCH timing adjustment info to the target cell.

__The soft handover algorithm

The WCDMA soft handover algorithm as described in the

3GPP TR 25.922 specifications differs slightly from the

IS 95A algorithm as used in cdmaOne, the standard for

North American cellular systems also based on CDMA.

Even though the significance of the latter cannot be

ignored this discussion is restricted to the analysis of the

WCDMA algorithm only.

Based on the Ec/I0 measurements of the set of cells

monitored, the mobile station decides which of three basic

actions to perform; it is possible to add, remove or replace

a node B in the active cell. These tasks are respectively

called Radio Link Addition and Radio Link Removal,

while the latter is Combined Radio Link Addition and

Removal. The example below is directly taken from the

original 3GPP specifications. Discussing this scenario

gives a good insight into the algorithm itself and forms an

introduction to the illustrating simulations included in the

next paragraph. This scenario can be based on a user

following a trajectory as shown below.

Figure: soft handover scenario

Delays in handovers can cause a user to penetrate deeply

in an adjacent cell and generate harmful interference

before the cell is added to the active cell.

Figure shows how the pilot signal strengths of the

different cells evolve in time.

Figure: WCDMA handover algorithm (picture taken

from )


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At the start of the scenario the user is connected to cell

number 1 which has the strongest pilot signal. Due to the

user moving or to slow fading the perception of the signal

strengths to the mobile user can change and following

actions are taken:

- Event A: cell 2 is added

- Event B: cell 1 is replaced with cell 3

- Event C: Cell 3 is removed from the active set

This example could be based on a mobile user following a

path similar to the picture below.

The main parameter in the soft handover algorithm is the

threshold for soft handover as will be shown in the

following chapters the value of this figure is a crucial

design parameter, it determines the amount of users being

in soft handover mode and hence influences the system

capacity and coverage. Roughly stated it is the maximum

difference in SIR two pilot signals can have so their cells

can coexist in the active set.

References:

[1] H.-G. Jeon, S.-K. Kwon, S. R. Kim, and C.-E. Kang,

“A call control scheme for soft handoff in CDMA cellular

systems,” in Proc. IEEE Int. Conf. Communications

(ICC), Atlanta, GA, 1998, vol. 2, pp. 999–1003.

[2] B. H. Cheung and C. M. Leung, “Network

configurations for seamless support of CDMA soft

handoffs between cell clusters,” IEEE J. Sel. Areas

Commun., vol. 15, no. 7, pp. 1276–1288, Jul. 1997.

[3] D. J. Lee and D. H. Cho, “Soft handoff initiation

control in CDMAcellular system,” in Proc. IEEE

Vehicular Technology Conf. (VTC), Amsterdam, The

Netherlands, 1999, pp. 2143–2147.

[4] A. J. Viterbi, A. M. Viterbi, K. S. Gilhousen, and E.

Zehavi, “Soft handoff extends CDMA cell coverage and

increases reverse link capacity,” IEEE J. Sel. Areas

Commun., vol. 12, no. 8, pp. 1281–1288, Oct. 1994.

[5] C. William and Y. Lee, “Overview of cellular

CDMA,” IEEE Trans. Veh. Technol., vol. 40, no. 2, pp.

291–302, Feb. 1991.


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[6] D. K. Kim and D. K. Sung, “Characterization of soft

handoff in CDMA systems,” IEEE Trans. Veh. Technol.,

vol. 48, no. 4, pp. 1195–1202, Jul. 1999.

[7] N. Shinagawa, T. Kobayashi, K. Nakono, and M.

Sengoku, “Teletraffic evaluation between a switch and

base stations in cellular systems with multiple-connection

capability for soft handoff,” in Proc. IEEE Vehicular

Technology Conf. (VTC), Amsterdam, The Netherlands,

1999, vol. 2, pp. 1243–1247.

[8] N. Zhang and J. M. Holtsman, “Analysis of a CDMA

soft-handoff algorithm,” IEEE Trans. Veh. Technol., vol.

47, no. 2, pp. 710–714, May 1998.

[9] S.-L. Su, J.-Y. Chen, and J. H. Huang, “Performance

analysis of soft handoff in CDMA cellular networks,”

IEEE J. Sel. Areas Commun., vol. 14, no. 9, pp. 1762–

1769, Dec. 1996.

Soft and Softer Handover in Communication Netwoks

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